Saint Ignatius principal, Patrick Ruff, takes questions from the press in front of Saint Ignatius College Preparatory. Saint Ignatius officials held a press conference on Wednesday, February 1, 2012, to announce the closure of the school following a stomach flu outbreak.

Officials at St. Ignatius College Preparatory in San Francisco closed the Jesuit high school until Monday after more than 20 percent of students came down with stomach flu.

A total of 325 students and 30 teachers have been hit by the virus, school officials said at a news conference Wednesday. Dozens of students went home sick Tuesday after suddenly becoming ill.

"This number of students represents a significant percentage of our community," said Patrick Ruff, principal of the 1,500-student school in the Outer Sunset.

Angela Yip, a St. Ignatius junior, said many students fell ill with little or no warning Tuesday.

"In just my last period, there were like three people who just left during the class," she said. "You'd walk into the bathroom and there would be three people vomiting in there."

Some students made it to the bathroom, some did not, she said. Yip didn't get sick until she was on her way home.

"I got really dizzy all of a sudden and then I just got sick," she said.

The norovirus apparently associated with the outbreak hits quickly, said Kevin McCormack, a spokesman for the California Pacific Medical Center in San Francisco. In a matter of minutes, he said, people can go from feeling a little under the weather to something far worse.

"This is the kind of thing that you usually see on a cruise ship, but it seems to be spreading more widely here," McCormack said.

McCormack said 16 children went to California Pacific's four emergency rooms in the city with norovirus-like symptoms Tuesday. A similar outbreak at two nursing homes last week in Marin County prompted officials to quarantine sick patients and transfer healthy ones.

Shelley Gordon, a doctor with California Pacific who studies infectious diseases, said the rapid spread of the stomach bug at St. Ignatius suggested that a large number of students were exposed to the virus at once.

"It sounds to me like it was a food handler or someone like that," she said. "It would be really hard for me to believe that it spread like this if one or two people brought it back from vacation."

Ruff said health officials had tested the St. Ignatius kitchen and water supply and found nothing amiss.

Custodians are using steam and bleach to deep-clean the school, Chawla said. Officials initially said St. Ignatius would be shut down only for Wednesday, but then extended the closure through the weekend to give crews time to sanitize the campus, said Paul Totah, a school spokesman.

After-school activities such as athletics and club meetings have also been canceled.